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首页> 外文学位 >Applications of a laser-driven superconducting switch to fundamental measurements and to low-frequency noise reduction in SQUID measurements.
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Applications of a laser-driven superconducting switch to fundamental measurements and to low-frequency noise reduction in SQUID measurements.

机译：激光驱动的超导开关在基本测量和SQUID测量中降低低频噪声中的应用。

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摘要

A SQUID is used to study laser-switches, 10-40 nm thick niobium microbridges which are driven into the normal state by laser pulses sent through a multimode optical fiber. Laser pulses induce a random change in the quantized flux state of the SQUID's input circuit. From the correlation between successive flux states, we have established that the laser-switch is able to follow 6 ns laser pulses. A technique based on flux quantization is used to measure precisely the inductance of superconducting circuit elements, as well as the magnetic penetration depth of niobium. Using a specially shielded toroidal solenoid, we have demonstrated that magnetic flux is quantized absolutely in superconducting circuits, refuting an assertion that fractional quanta of magnetic flux might exist. We observe that magnetic flux is trapped in the laser-switch occasionally if the laser's driving current is reduced to zero, but never if the laser beam is interrupted mechanically. This observation is consistent with the hypothesis that flux is trapped when the laser-switch cools through its transition in the presence of a stable modal interference pattern produced by the fiber.; The noise energy spectrum of a SQUID is white at high frequencies and is 1/f (inversely proportional to frequency) at low frequencies; the limiting behaviors intersect at the 1/f knee. We have demonstrated two techniques which improve the signal to noise ratio in low-frequency SQUID measurements by modulating the signal at a frequency above the 1/f knee. In the first technique, a double-pole double-throw network of laser-switches is interposed between a pickup coil and the SQUID's input coil. For a modulated signal, the 1/f knee is reduced by more than an order of magnitude from the unmodulated 1/f knee with no loss in signal amplitude. In the second technique, the SQUID's input coil and a pickup coil are connected in series with a single laser-switch modulated inductor. The 1/f knee is reduced by more than two orders of magnitude, but with a factor of {dollar}sim{dollar}40 loss in signal amplitude due to incomplete inductance modulation.
机译：SQUID用于研究10-40 nm厚的激光开关铌微桥，这些激光桥通过多模光纤发送的激光脉冲驱动进入正常状态。激光脉冲在SQUID输入电路的量化通量状态中引起随机变化。根据连续磁通状态之间的相关性，我们已经确定了激光开关能够跟随6 ns的激光脉冲。基于磁通量量化的技术用于精确测量超导电路元件的电感以及铌的磁渗透深度。通过使用特殊屏蔽的环形螺线管，我们证明了超导电路中的磁通量是绝对量化的，从而驳斥了磁通量分数量子可能存在的说法。我们观察到，如果将激光器的驱动电流减小到零，则磁通量偶尔会被捕获在激光器的开关中，但是如果激光束没有被机械地中断，则磁通量不会被捕获。这一观察结果与这样的假设是一致的：当在光纤产生的稳定模态干涉图样存在的情况下，当激光开关在其转换过程中冷却时，通量会被捕获。 SQUID的噪声能量谱在高频下为白色，而在低频下为1 / f（与频率成反比）。限制行为在1 / f膝盖处相交。我们已经展示了两种技术，它们可以通过以高于1 / f拐点的频率调制信号来改善低频SQUID测量中的信噪比。在第一种技术中，激光开关的双刀双掷网络插入在拾波线圈和SQUID的输入线圈之间。对于调制信号，1 / f拐点与未调制的1 / f拐点相比减小了一个数量级以上，而信号幅度没有损失。在第二种技术中，SQUID的输入线圈和拾波线圈与单个激光开关调制电感器串联连接。 1 / f拐点减小了两个数量级以上，但由于电感调制不完全，信号幅度损失了40倍。

著录项

  • 作者

    Cunningham, Charles Ernest.;

  • 作者单位

    Stanford University.;

  • 授予单位 Stanford University.;
  • 学科 Physics Condensed Matter.; Physics Optics.
  • 学位 Ph.D.
  • 年度 1992
  • 页码 277 p.
  • 总页数 277
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 光学;
  • 关键词

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